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Angular - AOT metadata errors

AOT metadata errors link

The following are metadata errors you may encounter, with explanations and suggested corrections.

Expression form not supported
Reference to a local (non-exported) symbol
Only initialized variables and constants
Reference to a non-exported class
Reference to a non-exported function
Function calls are not supported
Destructured variable or constant not supported
Could not resolve type
Name expected
Unsupported enum member name
Tagged template expressions are not supported
Symbol reference expected

Expression form not supported link

The compiler encountered an expression it didn't understand while evaluating Angular metadata.

Language features outside of the compiler's restricted expression syntax can produce this error, as seen in the following example:

      // ERROR
export class Fooish {  }

const prop = typeof Fooish; // typeof is not valid in metadata
  // bracket notation is not valid in metadata
  { provide: 'token', useValue: { [prop]: 'value' } };

You can use typeof and bracket notation in normal application code. You just can't use those features within expressions that define Angular metadata.

Avoid this error by sticking to the compiler's restricted expression syntax when writing Angular metadata and be wary of new or unusual TypeScript features.

Reference to a local (non-exported) symbol link

Reference to a local (non-exported) symbol 'symbol name'. Consider exporting the symbol.

The compiler encountered a referenced to a locally defined symbol that either wasn't exported or wasn't initialized.

Here's a provider example of the problem.

      // ERROR
let foo: number; // neither exported nor initialized

  selector: 'my-component',
  template:  ,
  providers: [
    { provide: Foo, useValue: foo }
export class MyComponent {}

The compiler generates the component factory, which includes the useValue provider code, in a separate module. That factory module can't reach back to this source module to access the local (non-exported) foo variable.

You could fix the problem by initializing foo .

      let foo = 42; // initialized

The compiler will fold the expression into the provider as if you had written this.

      providers: [
  { provide: Foo, useValue: 42 }

Alternatively, you can fix it by exporting foo with the expectation that foo will be assigned at runtime when you actually know its value.

      // CORRECTED
export let foo: number; // exported

  selector: 'my-component',
  template:  ,
  providers: [
    { provide: Foo, useValue: foo }
export class MyComponent {}

Adding export often works for variables referenced in metadata such as providers and animations because the compiler can generate references to the exported variables in these expressions. It doesn't need the values of those variables.

Adding export doesn't work when the compiler needs the actual value in order to generate code. For example, it doesn't work for the template property.

      // ERROR
export let someTemplate: string; // exported but not initialized

  selector: 'my-component',
  template: someTemplate
export class MyComponent {}

The compiler needs the value of the template property right now to generate the component factory. The variable reference alone is insufficient. Prefixing the declaration with export merely produces a new error, " Only initialized variables and constants can be referenced ".

Only initialized variables and constants link

Only initialized variables and constants can be referenced because the value of this variable is needed by the template compiler.

The compiler found a reference to an exported variable or static field that wasn't initialized. It needs the value of that variable to generate code.

The following example tries to set the component's template property to the value of the exported someTemplate variable which is declared but unassigned .

      // ERROR
export let someTemplate: string;

  selector: 'my-component',
  template: someTemplate
export class MyComponent {}

You'd also get this error if you imported someTemplate from some other module and neglected to initialize it there.

      // ERROR - not initialized there either
import { someTemplate } from './config';

  selector: 'my-component',
  template: someTemplate
export class MyComponent {}

The compiler cannot wait until runtime to get the template information. It must statically derive the value of the someTemplate variable from the source code so that it can generate the component factory, which includes instructions for building the element based on the template.

To correct this error, provide the initial value of the variable in an initializer clause on the same line .

      // CORRECTED
export let someTemplate = '<h1>Greetings from Angular</h1>';

  selector: 'my-component',
  template: someTemplate
export class MyComponent {}

Reference to a non-exported class link

Reference to a non-exported class <class name> . Consider exporting the class.

Metadata referenced a class that wasn't exported.

For example, you may have defined a class and used it as an injection token in a providers array but neglected to export that class.

      // ERROR
abstract class MyStrategy { }

  providers: [
    { provide: MyStrategy, useValue:  }

Angular generates a class factory in a separate module and that factory can only access exported classes. To correct this error, export the referenced class.

      // CORRECTED
export abstract class MyStrategy { }

  providers: [
    { provide: MyStrategy, useValue:  }

Reference to a non-exported function link

Metadata referenced a function that wasn't exported.

For example, you may have set a providers useFactory property to a locally defined function that you neglected to export.

      // ERROR
function myStrategy() {  }

  providers: [
    { provide: MyStrategy, useFactory: myStrategy }

Angular generates a class factory in a separate module and that factory can only access exported functions. To correct this error, export the function.

      // CORRECTED
export function myStrategy() {  }

  providers: [
    { provide: MyStrategy, useFactory: myStrategy }

Function calls are not supported link

Function calls are not supported. Consider replacing the function or lambda with a reference to an exported function.

The compiler does not currently support function expressions or lambda functions. For example, you cannot set a provider's useFactory to an anonymous function or arrow function like this.

      // ERROR
  providers: [
    { provide: MyStrategy, useFactory: function() {  } },
    { provide: OtherStrategy, useFactory: () => {  } }

You also get this error if you call a function or method in a provider's useValue .

      // ERROR
import { calculateValue } from './utilities';

  providers: [
    { provide: SomeValue, useValue: calculateValue() }

To correct this error, export a function from the module and refer to the function in a useFactory provider instead.

      // CORRECTED
import { calculateValue } from './utilities';

export function myStrategy() {  }
export function otherStrategy() {  }
export function someValueFactory() {
  return calculateValue();
  providers: [
    { provide: MyStrategy, useFactory: myStrategy },
    { provide: OtherStrategy, useFactory: otherStrategy },
    { provide: SomeValue, useFactory: someValueFactory }

Destructured variable or constant not supported link

Referencing an exported destructured variable or constant is not supported by the template compiler. Consider simplifying this to avoid destructuring.

The compiler does not support references to variables assigned by destructuring.

For example, you cannot write something like this:

      // ERROR
import { configuration } from './configuration';

// destructured assignment to foo and bar
const {foo, bar} = configuration;
  providers: [
    {provide: Foo, useValue: foo},
    {provide: Bar, useValue: bar},

To correct this error, refer to non-destructured values.

      // CORRECTED
import { configuration } from './configuration';
  providers: [
    {provide: Foo, useValue: configuration.foo},
    {provide: Bar, useValue: configuration.bar},

Could not resolve type link

The compiler encountered a type and can't determine which module exports that type.

This can happen if you refer to an ambient type. For example, the Window type is an ambient type declared in the global .d.ts file.

You'll get an error if you reference it in the component constructor, which the compiler must statically analyze.

      // ERROR
@Component({ })
export class MyComponent {
  constructor (private win: Window) {  }

TypeScript understands ambient types so you don't import them. The Angular compiler does not understand a type that you neglect to export or import.

In this case, the compiler doesn't understand how to inject something with the Window token.

Do not refer to ambient types in metadata expressions.

If you must inject an instance of an ambient type, you can finesse the problem in four steps:

  1. Create an injection token for an instance of the ambient type.
  2. Create a factory function that returns that instance.
  3. Add a useFactory provider with that factory function.
  4. Use @Inject to inject the instance.

Here's an illustrative example.

      // CORRECTED
import { Inject } from '@angular/core';

export const WINDOW = new InjectionToken('Window');
export function _window() { return window; }

  providers: [
    { provide: WINDOW, useFactory: _window }
export class MyComponent {
  constructor (@Inject(WINDOW) private win: Window) {  }

The Window type in the constructor is no longer a problem for the compiler because it uses the @Inject(WINDOW) to generate the injection code.

Angular does something similar with the DOCUMENT token so you can inject the browser's document object (or an abstraction of it, depending upon the platform in which the application runs).

      import { Inject }   from '@angular/core';
import { DOCUMENT } from '@angular/common';

@Component({  })
export class MyComponent {
  constructor (@Inject(DOCUMENT) private doc: Document) {  }

Name expected link

The compiler expected a name in an expression it was evaluating.

This can happen if you use a number as a property name as in the following example.

      // ERROR
provider: [{ provide: Foo, useValue: { 0: 'test' } }]

Change the name of the property to something non-numeric.

      // CORRECTED
provider: [{ provide: Foo, useValue: { '0': 'test' } }]

Unsupported enum member name link

Angular couldn't determine the value of the enum member that you referenced in metadata.

The compiler can understand simple enum values but not complex values such as those derived from computed properties.

      // ERROR
enum Colors {
  Red = 1,
  Blue = "Blue".length // computed

  providers: [
    { provide: BaseColor,   useValue: Colors.White } // ok
    { provide: DangerColor, useValue: Colors.Red }   // ok
    { provide: StrongColor, useValue: Colors.Blue }  // bad

Avoid referring to enums with complicated initializers or computed properties.

Tagged template expressions are not supported link

Tagged template expressions are not supported in metadata.

The compiler encountered a JavaScript ES2015 tagged template expression such as the following.

      // ERROR
const expression = 'funky';
const raw = String.raw`A tagged template ${expression} string`;
 template: '<div>' + raw + '</div>'

String.raw() is a tag function native to JavaScript ES2015.

The AOT compiler does not support tagged template expressions; avoid them in metadata expressions.

Symbol reference expected link

The compiler expected a reference to a symbol at the location specified in the error message.

This error can occur if you use an expression in the extends clause of a class.

Last reviewed on Mon Feb 28 2022
Angular - App shell

App shell link

Application shell is a way to render a portion of your application using a route at build time. It can improve the user experience by quickly launching a static rendered page (a skeleton common to all pages) while the browser downloads the full client version and switches to it automatically after the code loads.

This gives users a meaningful first paint of your application that appears quickly because the browser can render the HTML and CSS without the need to initialize any JavaScript.

Learn more in The App Shell Model.

Step 1: Prepare the application link

Do this with the following Angular CLI command:

      ng new my-app --routing

For an existing application, you have to manually add the RouterModule and defining a <router-outlet> within your application.

Step 2: Create the application shell link

Use the Angular CLI to automatically create the application shell.

      ng generate app-shell

For more information about this command, see App shell command.

After running this command you can see that the angular.json configuration file has been updated to add two new targets, with a few other changes.

      "server": {
  "builder": "@angular-devkit/build-angular:server",
  "defaultConfiguration": "production",
  "options": {
    "outputPath": "dist/my-app/server",
    "main": "src/main.server.ts",
    "tsConfig": "tsconfig.server.json"
  "configurations": {
    "development": {
      "outputHashing": "none",
    "production": {
      "outputHashing": "media",
      "fileReplacements": [
          "replace": "src/environments/environment.ts",
          "with": "src/environments/environment.prod.ts"
      "sourceMap": false,
      "optimization": true
"app-shell": {
  "builder": "@angular-devkit/build-angular:app-shell",
  "defaultConfiguration": "production",
  "options": {
    "route": "shell"
  "configurations": {
    "development": {
      "browserTarget": "my-app:build:development",
      "serverTarget": "my-app:server:development",
    "production": {
      "browserTarget": "my-app:build:production",
      "serverTarget": "my-app:server:production"

Step 3: Verify the application is built with the shell content link

Use the Angular CLI to build the app-shell target.

      ng run my-app:app-shell:development

Or to use the production configuration.

      ng run my-app:app-shell:production

To verify the build output, open dist/my-app/browser/index.html . Look for default text app-shell works! to show that the application shell route was rendered as part of the output.

Last reviewed on Mon Feb 28 2022
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Angular - Introduction to components and templates

Introduction to components and templates link

A component controls a patch of screen called a view . It consists of a TypeScript class, an HTML template, and a CSS style sheet. The TypeScript class defines the interaction of the HTML template and the rendered DOM structure, while the style sheet describes its appearance.

An Angular application uses individual components to define and control different aspects of the application. For example, an application could include components to describe:

  • The application root with the navigation links
  • The list of heroes
  • The hero editor

In the following example, the HeroListComponent class includes:

  • A heroes property that holds an array of heroes.
  • A selectedHero property that holds the last hero selected by the user.
  • A selectHero() method sets a selectedHero property when the user clicks to choose a hero from that list.

The component initializes the heroes property by using the HeroService service, which is a TypeScript parameter property on the constructor. Angular's dependency injection system provides the HeroService service to the component.

src/app/hero-list.component.ts (class)
      export class HeroListComponent implements OnInit {
  heroes: Hero[] = [];
  selectedHero: Hero | undefined;

  constructor(private service: HeroService) { }

  ngOnInit() {
    this.heroes = this.service.getHeroes();

  selectHero(hero: Hero) { this.selectedHero = hero; }

Angular creates, updates, and destroys components as the user moves through the application. Your application can take action at each moment in this lifecycle through optional lifecycle hooks, like ngOnInit() .

Component metadata link

The @Component decorator identifies the class immediately below it as a component class, and specifies its metadata. In the example code below, you can see that HeroListComponent is just a class, with no special Angular notation or syntax at all. It's not a component until you mark it as one with the @Component decorator.

The metadata for a component tells Angular where to get the major building blocks that it needs to create and present the component and its view. In particular, it associates a template with the component, either directly with inline code, or by reference. Together, the component and its template describe a view .

In addition to containing or pointing to the template, the @Component metadata configures, for example, how the component can be referenced in HTML and what services it requires.

Here's an example of basic metadata for HeroListComponent .

src/app/hero-list.component.ts (metadata)
  selector:    'app-hero-list',
  templateUrl: './hero-list.component.html',
  providers:  [ HeroService ]
export class HeroListComponent implements OnInit {
  /* . . . */

This example shows some of the most useful @Component configuration options:

Configuration options Details
selector A CSS selector that tells Angular to create and insert an instance of this component wherever it finds the corresponding tag in template HTML. For example, if an application's HTML contains <app-hero-list></app-hero-list> , then Angular inserts an instance of the HeroListComponent view between those tags.
templateUrl The module-relative address of this component's HTML template. Alternatively, you can provide the HTML template inline, as the value of the template property. This template defines the component's host view .
providers An array of providers for services that the component requires. In the example, this tells Angular how to provide the HeroService instance that the component's constructor uses to get the list of heroes to display.

Templates and views link

You define a component's view with its companion template. A template is a form of HTML that tells Angular how to render the component.

Views are typically organized hierarchically, allowing you to modify or show and hide entire UI sections or pages as a unit. The template immediately associated with a component defines that component's host view . The component can also define a view hierarchy , which contains embedded views , hosted by other components.

A view hierarchy can include views from components in the same NgModule and from those in different NgModules.

Template syntax link

A template looks like regular HTML, except that it also contains Angular template syntax, which alters the HTML based on your application's logic and the state of application and DOM data. Your template can use data binding to coordinate the application and DOM data, pipes to transform data before it is displayed, and directives to apply application logic to what gets displayed.

For example, here is a template for the Tutorial's HeroListComponent .

      <h2>Hero List</h2>

<p><em>Select a hero from the list to see details.</em></p>
  <li *ngFor="let hero of heroes">
    <button type="button" (click)="selectHero(hero)">

<app-hero-detail *ngIf="selectedHero" [hero]="selectedHero"></app-hero-detail>

This template uses typical HTML elements like <h2> and <p> . It also includes Angular template-syntax elements, *ngFor , {{hero.name}} , (click) , [hero] , and <app-hero-detail> . The template-syntax elements tell Angular how to render the HTML to the screen, using program logic and data.

  • The *ngFor directive tells Angular to iterate over a list

  • {{hero.name}} , (click) , and [hero] bind program data to and from the DOM, responding to user input. See more about data binding below.

  • The <app-hero-detail> element tag in the example represents a new component, HeroDetailComponent . The HeroDetailComponent defines the hero-detail portion of the rendered DOM structure specified by the HeroListComponent component.

    Notice how these custom components mix with native HTML.

Data binding link

Without a framework, you would be responsible for pushing data values into the HTML controls and turning user responses into actions and value updates. Writing such push and pull logic by hand is tedious, error-prone, and a nightmare to read, as any experienced front-end JavaScript programmer can attest.

Angular supports two-way data binding , a mechanism for coordinating the parts of a template with the parts of a component. Add binding markup to the template HTML to tell Angular how to connect both sides.

The following diagram shows the four forms of data binding markup. Each form has a direction: to the DOM, from the DOM, or both.

This example from the HeroListComponent template uses three of these forms.

src/app/hero-list.component.html (binding)
      <app-hero-detail [hero]="selectedHero"></app-hero-detail>
<button type="button" (click)="selectHero(hero)">
Data bindings Details
[hero] property binding Passes the value of selectedHero from the parent HeroListComponent to the hero property of the child HeroDetailComponent .
(click) event binding Calls the component's selectHero method when the user clicks a hero's name.
{{hero.name}} interpolation Displays the component's hero.name property value within the <button> element.

Two-way data binding (used mainly in template-driven forms) combines property and event binding in a single notation. Here's an example from the HeroDetailComponent template that uses two-way data binding with the ngModel directive.

src/app/hero-detail.component.html (ngModel)
      <input type="text" id="hero-name" [(ngModel)]="hero.name">

In two-way binding, a data property value flows to the input box from the component as with property binding. The user's changes also flow back to the component, resetting the property to the latest value, as with event binding.

Angular processes all data bindings once for each JavaScript event cycle, from the root of the application component tree through all child components.

Data binding plays an important role in communication between a template and its component, and is also important for communication between parent and child components.

Pipes link

Angular pipes let you declare display-value transformations in your template HTML. A class with the @Pipe decorator defines a function that transforms input values to output values for display in a view.

Angular defines various pipes, such as the date pipe and currency pipe. For a complete list, see the Pipes API list. You can also define new pipes.

To specify a value transformation in an HTML template, use the pipe operator ( | ).

      {{interpolated_value | pipe_name}}

You can chain pipes, sending the output of one pipe function to be transformed by another pipe function. A pipe can also take arguments that control how it performs its transformation. For example, you can pass the desired format to the date pipe.

      <!-- Default format: output 'Jun 15, 2015'-->
<p>Today is {{today | date}}</p>

<!-- fullDate format: output 'Monday, June 15, 2015'-->
<p>The date is {{today | date:'fullDate'}}</p>

<!-- shortTime format: output '9:43 AM'-->
<p>The time is {{today | date:'shortTime'}}</p>

Directives link

Angular templates are dynamic . When Angular renders them, it transforms the DOM according to the instructions given by directives . A directive is a class with a @Directive() decorator.

A component is technically a directive. However, components are so distinctive and central to Angular applications that Angular defines the @Component() decorator, which extends the @Directive() decorator with template-oriented features.

In addition to components, there are two other kinds of directives: structural and attribute . Angular defines a number of directives of both kinds, and you can define your own using the @Directive() decorator.

Just as for components, the metadata for a directive associates the decorated class with a selector element that you use to insert it into HTML. In templates, directives typically appear within an element tag as attributes, either by name or as the target of an assignment or a binding.

Structural directives link

Structural directives alter layout by adding, removing, and replacing elements in the DOM. The example template uses two built-in structural directives to add application logic to how the view is rendered.

src/app/hero-list.component.html (structural)
      <li *ngFor="let hero of heroes"></li>
<app-hero-detail *ngIf="selectedHero"></app-hero-detail>
Directives Details
*ngFor An iterative , which tells Angular to create one <li> per hero in the heroes list.
*ngIf A conditional , which includes the HeroDetail component only if a selected hero exists.

Attribute directives link

Attribute directives alter the appearance or behavior of an existing element. In templates they look like regular HTML attributes, hence the name.

The ngModel directive, which implements two-way data binding, is an example of an attribute directive. ngModel modifies the behavior of an existing element (typically <input> ) by setting its display value property and responding to change events.

src/app/hero-detail.component.html (ngModel)
      <input type="text" id="hero-name" [(ngModel)]="hero.name">

Angular includes pre-defined directives that change:

  • The layout structure, such as ngSwitch, and
  • Aspects of DOM elements and components, such as ngStyle and ngClass.

Learn more in the Attribute Directives and Structural Directives guides.

Last reviewed on Mon Feb 28 2022
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Angular - Introduction to modules

Introduction to modules link

Angular applications are modular and Angular has its own modularity system called NgModules . NgModules are containers for a cohesive block of code dedicated to an application domain, a workflow, or a closely related set of capabilities. They can contain components, service providers, and other code files whose scope is defined by the containing NgModule. They can import functionality that is exported from other NgModules, and export selected functionality for use by other NgModules.

Every Angular application has at least one NgModule class, the root module , which is conventionally named AppModule and resides in a file named app.module.ts . You launch your application by bootstrapping the root NgModule.

While a small application might have only one NgModule, most applications have many more feature modules . The root NgModule for an application is so named because it can include child NgModules in a hierarchy of any depth.

NgModule metadata link

An NgModule is defined by a class decorated with @NgModule() . The @NgModule() decorator is a function that takes a single metadata object, whose properties describe the module. The most important properties are as follows.

Properties Details
declarations The components, directives , and pipes that belong to this NgModule.
exports The subset of declarations that should be visible and usable in the component templates of other NgModules.
imports Other modules whose exported classes are needed by component templates declared in this NgModule.
providers Creators of services that this NgModule contributes to the global collection of services; they become accessible in all parts of the application. (You can also specify providers at the component level.)
bootstrap The main application view, called the root component , which hosts all other application views. Only the root NgModule should set the bootstrap property.

Here's a simple root NgModule definition.

      import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';
  imports:      [ BrowserModule ],
  providers:    [ Logger ],
  declarations: [ AppComponent ],
  exports:      [ AppComponent ],
  bootstrap:    [ AppComponent ]
export class AppModule { }

AppComponent is included in the exports list here for illustration; it isn't actually necessary in this example. A root NgModule has no reason to export anything because other modules don't need to import the root NgModule.

NgModules and components link

NgModules provide a compilation context for their components. A root NgModule always has a root component that is created during bootstrap but any NgModule can include any number of additional components, which can be loaded through the router or created through the template. The components that belong to an NgModule share a compilation context.

A component and its template together define a view . A component can contain a view hierarchy , which allows you to define arbitrarily complex areas of the screen that can be created, modified, and destroyed as a unit. A view hierarchy can mix views defined in components that belong to different NgModules. This is often the case, especially for UI libraries.

When you create a component, it's associated directly with a single view, called the host view . The host view can be the root of a view hierarchy, which can contain embedded views , which are in turn the host views of other components. Those components can be in the same NgModule, or can be imported from other NgModules. Views in the tree can be nested to any depth.

The hierarchical structure of views is a key factor in the way Angular detects and responds to changes in the DOM and application data.

NgModules and JavaScript modules link

The NgModule system is different from, and unrelated to, the JavaScript (ES2015) module system for managing collections of JavaScript objects. These are complementary module systems that you can use together to write your applications.

In JavaScript each file is a module and all objects defined in the file belong to that module. The module declares some objects to be public by marking them with the export key word. Other JavaScript modules use import statements to access public objects from other modules.

      import { NgModule } from '@angular/core';
import { AppComponent } from './app.component';
      export class AppModule { }

Learn more about the JavaScript module system on the web.

Angular libraries link

Angular loads as a collection of JavaScript modules. You can think of them as library modules. Each Angular library name begins with the @angular prefix. Install them with the node package manager npm and import parts of them with JavaScript import statements.

For example, import Angular's Component decorator from the @angular/core library like this.

      import { Component } from '@angular/core';

You also import NgModules from Angular libraries using JavaScript import statements. For example, the following code imports the BrowserModule NgModule from the platform-browser library.

      import { BrowserModule } from '@angular/platform-browser';

In the example of the simple root module above, the application module needs material from within BrowserModule . To access that material, add it to the @NgModule metadata imports like this.

      imports:      [ BrowserModule ],

In this way, you're using the Angular and JavaScript module systems together . Although it's easy to confuse the two systems, which share the common vocabulary of "imports" and "exports", you will become familiar with the different contexts in which they are used.

Learn more from the NgModules guide.

Last reviewed on Mon Feb 28 2022
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Angular - Next steps: tools and techniques

Next steps: tools and techniques link

After you understand the basic Angular building blocks, you can learn more about the features and tools that can help you develop and deliver Angular applications.

  • Work through the Tour of Heroes tutorial to get a feel for how to fit the basic building blocks together to create a well-designed application.
  • Check out the Glossary to understand Angular-specific terms and usage.
  • Use the documentation to learn about key features in more depth, according to your stage of development and areas of interest.

Application architecture link

  • The Main Concepts section located in the table of contents contains several topics that explain how to connect the application data in your components to your page-display templates, to create a complete interactive application.
  • The NgModules guide provides in-depth information on the modular structure of an Angular application.
  • The Routing and navigation guide provides in-depth information on how to construct applications that allow a user to navigate to different views within your single-page application.
  • The Dependency injection guide provides in-depth information on how to construct an application such that each component class can acquire the services and objects it needs to perform its function.

Responsive programming link

The template syntax and related topics contain details about how to display your component data when and where you want it within a view, and how to collect input from users that you can respond to.

Additional pages and sections describe some basic programming techniques for Angular applications.

  • Lifecycle hooks: Tap into key moments in the lifetime of a component, from its creation to its destruction, by implementing the lifecycle hook interfaces.
  • Observables and event processing: How to use observables with components and services to publish and subscribe to messages of any type, such as user-interaction events and asynchronous operation results.
  • Angular elements: How to package components as custom elements using Web Components, a web standard for defining new HTML elements in a framework-agnostic way.
  • Forms: Support complex data entry scenarios with HTML-based input validation.
  • Animations: Use Angular's animation library to animate component behavior without deep knowledge of animation techniques or CSS.

Client-server interaction link

Angular provides a framework for single-page applications, where most of the logic and data resides on the client. Most applications still need to access a server using the HttpClient to access and save data. For some platforms and applications, you might also want to use the PWA (Progressive Web App) model to improve the user experience.

  • HTTP: Communicate with a server to get data, save data, and invoke server-side actions with an HTTP client.
  • Server-side rendering: Angular Universal generates static application pages on the server through server-side rendering (SSR). This allows you to run your Angular application on the server in order to improve performance and show the first page quickly on mobile and low-powered devices, and also facilitate web crawlers.
  • Service workers and PWA: Use a service worker to reduce dependency on the network and significantly improve the user experience.
  • Web workers: Learn how to run CPU-intensive computations in a background thread.

Support for the development cycle link

  • CLI Command Reference: The Angular CLI is a command-line tool that you use to create projects, generate application and library code, and perform a variety of ongoing development tasks such as testing, bundling, and deployment.
  • Compilation: Angular provides just-in-time (JIT) compilation for the development environment, and ahead-of-time (AOT) compilation for the production environment.
  • Testing platform: Run unit tests on your application parts as they interact with the Angular framework.
  • Deployment: Learn techniques for deploying your Angular application to a remote server.
  • Security guidelines: Learn about Angular's built-in protections against common web-application vulnerabilities and attacks such as cross-site scripting attacks.
  • Internationalization: Make your application available in multiple languages with Angular's internationalization (i18n) tools.
  • Accessibility: Make your application accessible to all users.

File structure, configuration, and dependencies link

  • Workspace and file structure: Understand the structure of Angular workspace and project folders.
  • Building and serving: Learn to define different build and proxy server configurations for your project, such as development, staging, and production.
  • npm packages: The Angular Framework, Angular CLI, and components used by Angular applications are packaged as npm packages and distributed using the npm registry. The Angular CLI creates a default package.json file, which specifies a starter set of packages that work well together and jointly support many common application scenarios.
  • TypeScript configuration: TypeScript is the primary language for Angular application development.
  • Browser support: Make your applications compatible across a wide range of browsers.

Extending Angular link

  • Angular libraries: Learn about using and creating re-usable libraries.
  • Schematics: Learn about customizing and extending the CLI's generation capabilities.
  • CLI builders: Learn about customizing and extending the CLI's ability to apply tools to perform complex tasks, such as building and testing applications.
Last reviewed on Mon Feb 28 2022
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Angular - Introduction to services and dependency injection

Introduction to services and dependency injection link

Service is a broad category encompassing any value, function, or feature that an application needs. A service is typically a class with a narrow, well-defined purpose. It should do something specific and do it well.

Angular distinguishes components from services to increase modularity and reusability.

Ideally, a component's job is to enable only the user experience. A component should present properties and methods for data binding to mediate between the view and the application logic. The view is what the template renders and the application logic is what includes the notion of a model .

A component should use services for tasks that don't involve the view or application logic. Services are good for tasks such as fetching data from the server, validating user input, or logging directly to the console. By defining such processing tasks in an injectable service class , you make those tasks available to any component. You can also make your application more adaptable by injecting different providers of the same kind of service, as appropriate in different circumstances.

Angular doesn't enforce these principles. Instead, Angular helps you follow these principles by making it easy to factor your application logic into services. In Angular, dependency injection makes those services available to components.

Service examples link

Here's an example of a service class that logs to the browser console.

src/app/logger.service.ts (class)
      export class Logger {
  log(msg: any)   { console.log(msg); }
  error(msg: any) { console.error(msg); }
  warn(msg: any)  { console.warn(msg); }

Services can depend on other services. For example, here's a HeroService that depends on the Logger service, and also uses BackendService to get heroes. That service in turn might depend on the HttpClient service to fetch heroes asynchronously from a server.

src/app/hero.service.ts (class)
      export class HeroService {
  private heroes: Hero[] = [];

    private backend: BackendService,
    private logger: Logger) { }

  getHeroes() {
    this.backend.getAll(Hero).then( (heroes: Hero[]) => {
      this.logger.log(`Fetched ${heroes.length} heroes.`);
      this.heroes.push(...heroes); // fill cache
    return this.heroes;

Dependency injection (DI) link

Dependency injection (DI) is the part of the Angular framework that provides components with access to services and other resources. Angular provides the ability for you to inject a service into a component to give that component access to the service.

The @Injectable() decorator defines a class as a service in Angular and allows Angular to inject it into a component as a dependency . Likewise, the @Injectable() decorator indicates that a component, class, pipe, or NgModule has a dependency on a service.

  • The injector is the main mechanism. Angular creates an application-wide injector for you during the bootstrap process, and additional injectors as needed. You don't have to create injectors.

  • An injector creates dependencies and maintains a container of dependency instances that it reuses, if possible.

  • A provider is an object that tells an injector how to obtain or create a dependency

For any dependency that you need in your app, you must register a provider with the application's injector, so that the injector can use the provider to create new instances. For a service, the provider is typically the service class itself.

A dependency doesn't have to be a service —it could be a function, for example, or a value.

When Angular creates a new instance of a component class, it determines which services or other dependencies that component needs by looking at the constructor parameter types. For example, the constructor of HeroListComponent needs HeroService .

src/app/hero-list.component.ts (constructor)
      constructor(private service: HeroService) { }

When Angular discovers that a component depends on a service, it first checks if the injector has any existing instances of that service. If a requested service instance doesn't yet exist, the injector makes one using the registered provider and adds it to the injector before returning the service to Angular.

When all requested services have been resolved and returned, Angular can call the component's constructor with those services as arguments.

The process of HeroService injection looks something like this.

Providing services link

You must register at least one provider of any service you are going to use. The provider can be part of the service's own metadata, making that service available everywhere, or you can register providers with specific modules or components. You register providers in the metadata of the service (in the @Injectable() decorator), or in the @NgModule() or @Component() metadata

  • By default, the Angular CLI command ng generate service registers a provider with the root injector for your service by including provider metadata in the @Injectable() decorator. The tutorial uses this method to register the provider of HeroService class definition.

     providedIn: 'root',

    When you provide the service at the root level, Angular creates a single, shared instance of HeroService and injects it into any class that asks for it. Registering the provider in the @Injectable() metadata also allows Angular to optimize an app by removing the service from the compiled application if it isn't used, a process known as tree-shaking .

  • When you register a provider with a specific NgModule, the same instance of a service is available to all components in that NgModule. To register at this level, use the providers property of the @NgModule() decorator.

      providers: [
  • When you register a provider at the component level, you get a new instance of the service with each new instance of that component. At the component level, register a service provider in the providers property of the @Component() metadata.

    src/app/hero-list.component.ts (component providers)
      selector:    'app-hero-list',
      templateUrl: './hero-list.component.html',
      providers:  [ HeroService ]

For more detailed information, see the Dependency Injection section.

Last reviewed on Mon Feb 28 2022
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